What is a Vacuum Leak?

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Vacuum Leak:

The pumping of an engine’s pistons creates a vacuum within the cylinders, where the vacuum is filled by air drawn in through the intake system. If leaks develop in hoses or engine parts downstream from the air intake, it results in additional air goes into the engine that’s not being “read” by the engine computer, which determines what quantity of fuel to inject into the cylinders supported by the number of air coming in.

The result’s an incorrect air-fuel mixture that can create some problems, like hard starting, a rough idle, hesitation, misfiring, or a call in fuel economy. Issues that would, without proper diagnosis, seem to own other causes, like incorrect ignition timing or fouled spark plugs. A vacuum leak can trigger a “check engine” light because the equipment is running too “lean,” resulting in a greater mixture of air than is good.

Vacuum leaks can develop in many locations, including the manifold, throttle body evaporative emissions system, power brake booster, and several other places, because some vehicles have a large number of vacuum hoses.

Under normal conditions, all of the engine’s intake air is routed through the throttle body. Inside the throttle body is where the mass air-flow sensor accurately measures the number of air going into the engine.

When a vacuum leak occurs, some air enters the manifold without passing through the throttle body. The ECU, meanwhile, injects the quantity of fuel necessary based only on the measured air-flow.

This ends up in a lean fuel-air mixture, which then ends up in erratic idling and lackluster low-speed performance.


O2 (Oxygen) Sensor:

Recall that the oxygen sensor is a component of the engine management system that measures residual oxygen within the exhaust gases.

Because of the surplus air intake, this data won’t tally with ECU-measured air-flow, which then ends up in the “Check Engine” light being triggered.

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